The present invention relates to a reliable and low-cost semiconductor device, or more in particular to an IC (Integrated Circuit) card or a multichip module.
Conventional techniques for IC cards are described in xe2x80x9cInformation Processing Handbookxe2x80x9d, first edition, pp. 302-304, compiled by Information Processing Society of Japan and published by Ohm, May 30, 1989. The same reference contains at pp. 242-244 also the description of techniques for packaging the IC card. The structure of IC cards is described in xe2x80x9cIC Cardsxe2x80x9d, first edition, p. 33, compiled by The Institute of Electronics, Information and Communication Engineers and published by Ohm, May 25, 1990. Also, an IC card using a thin LSI is disclosed in JP-A-3-87299, Apr. 12, 1991.
FIGS. 1, 2 and 3 are sectional views showing configurations of IC cards.
In FIG. 1 showing a conventional IC card configuration, a chip 211 is bonded to a portion having a contact 210, connected to a printed board 212 by a bonding wire 216, and sealed by resin 215. This module is embedded in a center core 213 of a hard material. The card surface is covered with an oversheet 209 and an oversheet 214.
FIG. 2 shows another example of the prior art. A semiconductor chip is bonded to a substrate 207 by a adhesive agent 207a. Due to a thick silicon substrate 217, however, the semiconductor is connected by being bonded to the substrate 207 by the adhesive 207a while absorbing the unevenness through a bonding wire 218.
In the example shown in FIG. 3, an IC 6 has a great thickness of about 200 to 400 xcexcm. This bulk IC 6 is bonded to a card board 8 by an adhesive 7. Since the bulk IC is thick, however, the uneven wiring patterns on the IC and a substrate wiring 10 are connected by a wire bonding 9. In this case, the bulk IC is easily subjected to bending stress and therefore stress relaxation is required. Also, in view of the limited sizes of the bulk IC, the structural requirement for improving the bending strength and the difficulty of reducing the number of wire-adhesive steps, the cost tends to increase.
JP-A-3-87299 (Apr. 12, 1991) has rendered well known an IC card configuration in which an IC module having a very thin LSI ground very thin leaving active elements is fitted in a surface recess.
This conventional configuration is shown in FIG. 4. A semiconductor element 204 is bonded on a substrate 207 by an adhesive 207a. A wiring 208 for connecting semiconductor elements is connected to a conductive pad 202 by way of a through hole 203. This conductive pad 202 is further connected to the wiring on the substrate 207 by conductive paste 201.
The problem of this configuration, as shown in FIG. 4, is that an adhesive layer is in direct contact with the lower surface of the semiconductor element 204 such as a transistor and ionic contaminants easily intrude the semiconductor element, thereby extremely deteriorating the reliability. FIG. 5 is a diagram showing a problem specific to an IC card configured using a thin LSI disclosed in JP-A-3-87299 (Apr. 12, 1991). A thin LSI 41 mounted on a thick card substrate 42 is subjected to tensile or compressive stress on the front and reverse surfaces when the card substrate 42 is bent, thereby exerting a large stress on the LSI chip. The resulting thin structure and low mechanical strength under a large stress causes the IC to be easily broken by the stress. This gives a rise to a new problem of a considerably reduced reliability.
As described above, the IC card using a thin LSI layer including a thin semiconductor element is easily affected by ionic contaminants. Also, the thinness leads to a low mechanical strength. In the conventional IC cards using a bulk LSI, a bulk IC chip is attached on a thin, easy-to-bend card and wire-bonded. Therefore, the IC is easily broken and is low in reliability. Further, the increased number of packaging steps makes a cost reduction difficult.
The object of the present invention is to solve the above-mentioned problems of the prior art and to provide a semiconductor circuit, or more in particular a IC card or multichip module high in reliability and low in cost.
According to one aspect of the invention for solving the above-mentioned problems, there is provided a thin-film semiconductor device comprising at least a semiconductor element and a wiring, wherein a thin film of a protective insulating material for protecting the semiconductor element is formed on the lower surface thereof in contact with the semiconductor element, and the surface of the protective insulating film is bonded to other substrate.
According to another aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the thin-film semiconductor circuit includes a thin film of a semiconductor circuit formed on a silicon-on-insulator (hereinafter referred to as SOI) wafer, another substrate coupling the thin-film semiconductor circuit on the opposite side formed with the semiconductor circuit, and a hardenable conductive material for connecting the wiring prepared on the substrate and the wiring of the thin-film semiconductor circuit.
According to a third aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor de vice wherein the thin-film semiconductor circuit is taken out of the main surface of the SOI wafer with the insulator layer inward thereof as a boundary.
According to another aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the thin-film semiconductor circuit and the other substrate are bonded by a rubber-like adhesive.
According to another aspect of the invention, there is provided a semiconductor device wherein the main surface is bonded to another support substrate, and then the SOI wafer substrate is ground or etched off.
According to still another aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the other substrate for coupling the thin-film semiconductor circuit is of a flexible card shape.
According to a further aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the other support substrate is flexible.
According to a still further aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the thin-film semiconductor circuit and the support substrate are bonded to each other by an adhesive separable under ultraviolet ray.
According to an even further aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the wiring by a liquid conductive material is a printed wiring with a rotary drum.
According to another aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the thin-film semiconductor circuit is located at the center of the same depth from the front and reverse surfaces of the other substrate.
According to still another aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the thin-film semiconductor circuit is bonded to one substrate and covered by being bonded to the other substrate of the same thickness.
According to a further aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the thin-film semiconductor circuit is formed with a wafer other than SOI.
According to a still further aspect of the invention for solving the above-mentioned problems, there is provided a card-like semiconductor device with a thin IC chip built in the card, wherein the thickness of the IC chip is 110 microns or less for the completed card thickness of 760 microns or more, 19 microns or less for the completed card thickness of 500 microns or more, and 4 microns or less for the completed card thickness of 250 microns or more.
According to an even further aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the thickness of the IC chip is at least 4 microns or less for the completed IC card thickness 250 microns or less.
According to another aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the thin IC chip is located at the center along the thickness of the card.
According to still another aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the thin IC chip is held between two or more card substrates.
According to a further aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the thin IC chip is preferably bonded in such a configuration as to be held between card substrates by a flexible adhesive.
According to a still further aspect of the invention for solving the above-mentioned problems, there is provided a semiconductor device wherein the protective insulating material is silicon nitride.
The above-mentioned object can be achieved by providing a semiconductor device in which a semiconductor circuit prepared from a silicon-on-insulator (hereinafter referred to as SOI) wafer in thin film form is coupled to other substrate, to which a wiring prepared in advance and a wiring for the above-mentioned semiconductor circuit are connected by a liquid conductive material, and this liquid conductive material is then hardened.
This thin-film semiconductor circuit can be coupled to the other substrate after being taken out of the main surface with the insulator layer inward of the SOI wafer as a boundary. Specifically, the thin-film semiconductor circuit can be prepared by coupling the main surface of the semiconductor circuit formed of the SOI wafer to another support substrate and then by grinding or etching off the substrate of the SOI wafer.
The thin-film semiconductor circuit is preferably coupled to the other substrate by a rubber-like adhesive.
Also, the other substrate to which the thin-film semiconductor circuit is coupled preferably is card-shaped and flexible.
The other support substrate is also preferably flexible.
The thin-film semiconductor circuit and the other support substrate are bonded to each other by the use of an adhesive (hereinafter referred to as the ultraviolet-separating adhesive) the bonding strength of which decreases under ultraviolet ray, thereby facilitating separation of the other support substrate during the process.
Further, the liquid conductive material is effectively applied on the wiring by printing using a rotary drum.
Furthermore, the IC card can be easily fabricated by providing the thin-film semiconductor circuit placed at the center of the same depth from the front and reverse surfaces of the substrate, or more specifically, by bonding the thin-film semiconductor circuit to one substrate and then covering it by bonding with another substrate of the same thickness.
Although the foregoing description refers to the case of using a thin-film semiconductor circuit prepared on an SOI wafer, a similar effect of course is obtained by the use of a thin-film semiconductor circuit formed on a wafer other than SOI.
The above-mentioned method for attaching a protective insulating layer on the reverse surface of a thin semiconductor element causes the protective insulating film to prevent intrusion of ionic contaminants from the reverse side of the semiconductor element nearest to the external environment, and therefore the reliability is improved. As a result, an IC card with an improved durability can be fabricated even when a thin LSI is bonded to a substrate using a low-cost organic adhesive generally containing considerable ionic impurities.
The use of silicon nitride as the protective insulating film with a large thermal expansion coefficient suppresses the curl of the thin LSI film due to the internal residual stress, thereby contributing to an improved reliability of the IC card.
When an SOI wafer is used, the inner insulator layer acts as a stopper in processing, so that a thin film IC can be prepared uniformly with a high reproducibility. The thin-film IC thus prepared is 5 to 10 xcexcm in thickness. This much thin IC is resistive to bend, and when bonded with a flexible adhesive to a thin substrate like an IC card, exhibits a high bending strength leading to a high reliability.
Also, the thin-film IC, which itself tends to break easily, is preferably mounted on a support substrate in advance to add stability. The support substrate to which the IC is coupled can be reliably removed at low temperatures by using an adhesive separable under ultraviolet ray. The thin-film IC attached to the card is so thin that wiring is possible with a conductive paste between the substrate and the IC. As compared with the conventional wire-bonding method using a gold wire, the method according to the invention is suited to mass production of flat, thin IC cards with low material cost.
The above-mentioned methods are applicable not only to the IC card but also to similar packaging of ICs as well as to multichip packaging.
Consider the section of a bent portion of a flat IC card. The curved surface develops an elongation and the reverse side thereof a compression. Under this condition, the central part of the section of the IC card is under a small stress free of compression. If a thin IC chip exists at this portion, such an IC chip is subjected to less stress. The IC chip is preferably thin. In the case where the card is thick, however, the card rigidity increases the critical curvature, thereby making it harder to bend. For this reason, the IC chip can be thick to some degree. In the case where the IC card is thin and easy to bend, on the other hand, the IC chip must also be reduced in thickness in order to relax the stress on the IC chip. In preparing a thin IC, the thinner the IC, the higher accuracy is required of fabrication devices. Changing the required IC chip thickness in accordance with the thickness of the IC card, therefore, is very important from the economic viewpoint and also for securing the required reliability. In this way, the correlationship between the thicknesses of the IC card and the IC chip is recognized to produce an economic and reliable IC card. Specifically, the thickness of the IC chip is rendered 110 microns or less for a card in completed form having the thickness of 760 microns or more, 19 microns or less for a card in completed form having the thickness of 500 microns or more, and 4 microns or less for a card in completed form having the thickness of 250 microns or more.